This invention relates to using optical properties of a medium to represent information.
One such use is shown in U.S. Pat. No. 4,682,954 (Richard C. Cook, inventor) which describes a method for enciphering and deciphering printed text and graphic matter by forming a modulo-two sum with a printed random key using a multi-step photographic or photocopy process to decipher. Cook employs numerous clear (light transmitting) regions and opaque (light absorbing) regions on two transparencies to represent binary information derived from light and dark regions in original printed matter.
U.S. Pat. No. 4,586,711 "Matching Card Game Employing Randomly-Coded Monochromatic Images" (Simon N. Winters and Yiu T. Chu, inventors) describes a system in which a message is hidden in a pair of images having clear and opaque regions and is recovered by overlaying one image on the other. The image to be hidden is masked by a process that in effect "paints" over the black areas of the image with a random fine-grain black-and-white "paint", and "paints" over the white areas of the image with another random fine-grain black-and-white "paint" that has a mathematically different but visually similar statistical behavior from the first "paint". The matching overlay, which must be on a transparency, is prepared in the Winters/Chu process by inverting the random "paint" over the black areas so that when the two images are overlaid, the black portions of the original image will appear solid black in the overlaid pair. The white portions of the original image are recorded in the matching overlay as before, but using a different and unrelated random pattern in the "paint". Therefore, the white portions of the original will appear in the overlaid pair as the sum of two random "paints", namely a dark gray.